Controllable assembly, characterization and catalytic properties of a new Strandberg-type organophosphotungstate

Article abstract of DOI:10.1039/c5dt04412j

Using phenylphosphonic acid, simple tungstate and copper(ii) compounds as starting materials, an organic-inorganic hybrid Strandberg-type organophosphotungstate, {[(Cu(H₂O)(μ-bipy))₂(C₆H₅PO₃)₂W₅O₁₅]}_n (bipy = 4,4′-bipyridyl) (1), was assembled successfully under hydrothermal conditions and characterized by physico-chemical and spectroscopic methods. Compound 1 represents the first example of a transition metal complex modified organophosphotungstate cluster. In the crystal structure of compound 1, the polymeric 1-D {Cu-bipy}_n chains are interconnected by [(C₆H₅PO₃)₂W₅O₁₅]^4- (abbreviated as {(C₆H₅P)₂W₅}) units into a 3-D framework. A hollow Keggin isopolytungstate [H₂W₁₂O₄₀]^6- ({W₁₂})-Cu(ii) coordination polymer, {[Cu(bipy)₂((μ-bipy)Cu(bipy))₂(H₂W₁₂O₄₀)]·12H₂O}_n (2), was obtained at different molar ratios of the starting materials and pH. The two Cu(ii) coordination polymers exhibit good acid-catalytic activity for the synthesis of cyclohexanone ethylene ketal. Their fluorescence properties were studied.

Full text of DOI:10.1039/c5dt04412j

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Controllable assembly, characterization and
catalytic properties of a new Strandberg-type
Cite this: DOI: 10.1039/c5dt04412j
organophosphotungstate†
Hong-Xin Ma,a Jing Du,a Zai-Ming Zhu,*a Ting Lu,a Fang Su*a,b and Lan-Cui Zhang*a
Using phenylphosphonic acid, simple tungstate and copper(II) compounds as starting materials, an
organic–inorganic hybrid Strandberg-type organophosphotungstate, {[(Cu(H2O)(μ-bipy))2(C6H5PO3)2W5O15]}n
(bipy = 4,4’-bipyridyl) (1), was assembled successfully under hydrothermal conditions and characterized
by physico-chemical and spectroscopic methods. Compound 1 represents the first example of a
transition metal complex modified organophosphotungstate cluster. In the crystal structure of compound 1,
4−
the polymeric 1-D {Cu-bipy}n chains are interconnected by [(C6H5PO3)2W5O15
]
(abbreviated as
6−
{(C6H5P)2W5}) units into a 3-D framework. A hollow Keggin isopolytungstate [H2W12O40
]
({W12})-Cu(II)
Received 8th November 2015,
Accepted 8th December 2015
coordination polymer, {[Cu(bipy)2((μ-bipy)Cu(bipy))2(H2W12O40)]·12H2O}n (2), was obtained at different
molar ratios of the starting materials and pH. The two Cu(II) coordination polymers exhibit good
acid-catalytic activity for the synthesis of cyclohexanone ethylene ketal. Their fluorescence properties
were studied.
DOI: 10.1039/c5dt04412j
www.rsc.org/dalton
(C6H5P)2W5), was synthesized by Pope in 1990,5 while the
Strandberg-type {(RP)2W5} derivative with a transition metal
Introduction
Organic or organometallic compounds, such as organosilicon, complex modified or extended structure has not been reported
organophosphorus, and organotin, can be grafted on poly- so far. Our recent research work shows that the Strandberg-
anions, forming many new polyoxometalate (POM)-based type {(RP)2Mo5} POMs, [(Cu(H2O))2(μ-bipy)2(C6H5PO3)2Mo5O15]n
hybrid materials.1 Indeed, organic components are not only (abbreviated as bipy-Cu-(C6H5P)2Mo5) and [(Cu(H2O)2)2(μ-bipy)
used as structure-directing agents in preparing many (C6H5PO3)2Mo5O15]n, exhibit better catalytic activities than
materials, but can also be directly embedded into the those of {P2Mo5} POMs with central inorganic phosphorus
inorganic skeleton of the materials.2 So far, a few studies atoms,2c suggesting that the RP group plays an important role
about POMs containing an organophosphorus (RP) group in the catalytic activity of the Strandberg-type POMs. In
have been reported.1f,2,3 In particular, a RP group as a addition, this also shows that the Strandberg-type POMs, as
central unit has been introduced into the Strandberg-type small molecules of the POM family, may have unpredictable
POM skeleton successfully.2b,3d,e,4 However, most of the properties and potential applications. Some performances,
POM-RP compounds are of the organophosphomolybdate such as catalytic properties and photoluminescence of the in-
{(RP)2Mo5} series, the organophosphotungstate {(RP)2W5} organic–organic hybrid POM materials can be enhanced after
series has been rarely reported. As far as we know, only one the introduction of a transition metal and an organic com-
example of Strandberg-type polyoxotungstate with the central ponent into the structure.2c Through a variety of experiments,
RP, i.e., ((C4H9)3NH)4[(C6H5PO3)2W5O15] (abbreviated as TBA- we found that the synthesis of the {(RP)2W5} series is more
difficult than that of the {(RP)2Mo5} series. Therefore, the
design of transition metal complex modified {(RP)2W5}
hybrids is a meaningful and challenging work. Herein, a new
crystalline 3-D Cu(II) coordination polymer composed of
aSchool of Chemistry and Chemical Engineering, Liaoning Normal University,
Dalian 116029, China. E-mail: zhanglancui@lnnu.edu.cn, chemzhu@sina.com;
Fax: +86 411 82158559; Tel: +86 411 82156550
bipy units and Strandberg-type organophosphotungstate
4−
bCentre of Analytical Test, Liaoning Normal University, Dalian 116029, China.
[(C6H5PO3)2W5O15
]
polyanions was successfully synthesized
E-mail: sufang861218@163.com
†Electronic supplementary information (ESI) available: ORTEP views of com-
by changing the synthetic conditions. And another Cu(II)
coordination polymer containing the Keggin isopolytungstate
pounds 1 and 2; IR, XRPD, TG-DTA, emission spectra and catalytic results.
CCDC 1417285 and 1417286. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5dt04412j
6−
[H2W12O40
]
anion and bipy unit was unexpectedly
obtained. The influence of the molar ratio of the starting
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materials, and the pH of the mixture during the preparation of 32% based on W). Elemental analysis, Calc. for
the crystalline structure was discussed in detail. Additionally, C60H74O52N12Cu3W12: C 17.2, H 1.8, N 4.0, Cu 4.5, W 52.6%;
the solid-state fluorescence properties and catalytic activity of Found: C 17.3, H 1.9, N 4.1, Cu 4.5, W 52.8%. IR (KBr pellet),
the compounds were also studied.
cm−1: 3456(m), 3136(m), 1614(m), 1399(s), 1222(w), 1134(w),
919(w), 864(w), 775(m), 632(w).
Single-crystal X-ray crystallography
Experimental
The structures were solved by direct methods and refined by
full-matrix least-squares fitting on F2 using SHELXTL-2014
package. An empirical absorption correction was applied using
the SADABS program.6,7 Crystal data and structure refinement
parameters of compounds 1 and 2 are listed in Table 1. H
atoms on C atoms were added in calculated positions. In com-
pound 2, eleven lattice water molecules could be accurately
assigned from the residual electron peaks, and another lattice
water molecule was directly included in the molecular formula
based on the elemental analysis and TG analysis. All hydrogen
atoms on water molecules were directly included in the final
molecular formula. The selected bond lengths and angles and
hydrogen bonds are listed in Tables S1–S3.† CCDC reference
numbers: 1417285 and 1417286.
Reagents and general methods
All reagents were commercially available and were used as
received. TBA-(C6H5P)2W5 was prepared and characterized
according to the reported procedures.5 Elemental analyses
were performed on a Vario El cube elemental analyzer (C, H
and N) and a Prodigy XP emission spectrometer (P, Cu and W).
Single crystal X-ray diffraction data were collected on a Bruker
Smart APEX IIX-diffractometer equipped with graphite-mono-
chromated Mo Kα radiation (λ = 0.71073 Å). The infrared
spectra (IR) were recorded on KBr pellets with a Bruker AXS
TENSOR-27 FTIR spectrometer in the range of 4000–400 cm−1
.
X-ray powder diffraction (XRPD) data were obtained on a
Bruker AXS D8 Advance diffractometer using Cu Kα radiation
(λ = 1.5418 Å) with a step size of 0.02° in the 2θ range from 5 to
60°. Thermal analyses (TGA-DTA) were carried out on a Pyris
Diamond TG-DTA thermal analyzer in air at a heating rate of
10 °C min−1. The solid UV spectra were recorded on a Perkin-
Elmer Lambda 35 UV-visible spectrophotometer. The photo-
luminescence properties were determined on a HITACHI
F-7000 fluorescence spectrophotometer in the solid state at
room temperature. The yield of cyclohexanone ethylene ketal
was confirmed on a JK-GC112A gas chromatograph.
Catalytic experiment
Following the method mentioned in our previous work,1f,2c the
typical procedure for the acid catalytic synthesis is: the catalyst
(e.g. compound 1) was added to a mixture of cyclohexanone,
glycol and 10 mL of cyclohexane in a 100 mL three-necked
round-bottom flask fitted with a Dean–Stark apparatus to
remove the water continuously from the reaction mixture. For
the insoluble catalyst, the catalytic reaction undergoes a
heterogeneous process, after completing the reaction, the cata-
lyst could be separated from the organic phase containing the
product through decantation. Furthermore, the reusabilities of
Synthesis of compounds
Compound 1. CuCl2·2H2O (0.03 g, 0.18 mmol) and
Na2WO4·2H2O (0.18 g, 0.54 mmol) were dissolved in 20 mL of
distilled water under stirring at room temperature for 20 min.
Then C6H5PO3H2 (0.10 g, 0.64 mmol) was added to the above
solution under stirring, and the pH of the mixture was
adjusted to 2–3 with 4 mol L−1 HCl. Finally, bipy (0.05 g,
0.32 mmol) was added. The resulting mixture was transferred
to a 30 mL Teflon-lined autoclave and kept at 150 °C
for 3 days, and then blue block crystals were isolated
(yield: ca. 40% based on W). Elemental analysis, Calc. for
C32H30O23N4Cu2P2W5: C 19.7, H 1.55, N 2.9, P 3.2, Cu 6.5,
W 47.2%; Found: C 19.65, H 1.6, N 3.0, P 3.1, Cu 6.4, W 47.2%.
IR (KBr pellet), cm−1: 3447(w), 3115(m), 1613(m), 1530(w),
1408(m), 1313(w), 1223(w), 1160(m), 1128(s), 1033(m), 964(m),
939(s), 887(s), 812(m), 722(s), 686(s), 560(m), 512(m).
Table 1 Crystal and refinement data for 1 and 2
1
2
Formula
Formula weight
T/K
Wavelength/Å
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
C32H30Cu2N4O23P2W5
1946.87
296(2)
0.71073
Orthorhombic
Fdd2
19.0050(11)
42.763(2)
10.6834(6)
90.00
C60H74O52N12Cu3W12
4192.04
296(2)
0.71073
Monoclinic
P21/c
12.796(4)
25.625(7)
16.112(3)
90
β/°
90.00
90.00
8682.6(9), 8
2.979, 7136
1.082
119.955(16)
90
4577(2), 2
3.042, 3816
1.047
Compound 2. CuCl2·2H2O (0.09 g, 0.54 mmol) and
Na2WO4·2H2O (0.18 g, 0.54 mmol) were dissolved in 20 mL of
distilled water under stirring at room temperature for 20 min.
Then C6H5PO3H2 (0.10 g, 0.64 mmol) was added to the above
solution under stirring, and the pH of the mixture was
adjusted to 3–4 with 4 mol L−1 HCl. Finally, bipy (0.15 g,
0.96 mmol) was added. The resulting mixture was transferred
to a 30 mL Teflon-lined autoclave and kept at 150 °C for 3
γ/°
V/Å3, Z
Dc/g cm−3, F000
GOF
Reflections collected
Unique data, Rint
θ Range (°)
R1(I > 2σ(I))a
wR2 (all data)a
10 739
22 894
3340, 0.0358
1.905 to 24.995
0.0238
8059, 0.0372
1.661 to 25.00
0.0854
0.0625
0.1965
days, and then blue block crystals were obtained (yield: ca. a R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2
.
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compounds 1 and 2 were studied through three cycles under pounds, compared with the {(C6H5P)2Mo5} series, are more sen-
the optimum reaction conditions. The obtained ketal products sitive to the reaction conditions, such as pH and the molar ratio
were tested by gas chromatography.
of the precursors.
Structural analysis
Single crystal X-ray diffraction analysis reveals that compound
1 and bipy-Cu-{(C6H5P)2Mo5} are isostructural. The main struc-
Results and discussion
Synthesis
tural characteristics of 1 are also considered as a Cu(II) coordi-
4−
nation polymer, and consists of [(C6H5PO3)2W5O15
]
The organophosphorus group, as a central unit, is difficult to
be introduced into the POM system because of the hydro-
phobicity of the organic components and the spatial configuration
of the POM, although some {(RP)2Mo5} compounds have been
obtained. In particular, the preparation of the {(RP)2W5} series
is more difficult. Moreover, it is another challenge to harvest
the crystals and further determine their structures. According
to the synthetic methods of bipy-Cu-(C6H5P)2Mo5 reported in
our recent work,2c a similar synthetic protocol was adopted
except that Na2WO4 was used in place of Na2MoO4 in order to
obtain the crystalline bipy-Cu(II) modified Strandberg-type
organophosphotungstate. Under identical conditions, i.e., the
molar ratio of W/P/Cu/bipy is 2.5 : 1.0 : 1.0 : 1.0, pH is 3–4
(Scheme 1, step ③), unfortunately, only powders were
obtained. When the molar ratio of W/P/Cu/bipy was increased
to 3.0 : 3.6 : 1.0 : 1.8 (step ②), no crystal was gained. While
attempting to decrease the molar ratio of W/P to 1.0 : 1.2
(step ①), the blue block crystal (compound 2) was obtained
unexpectedly, the RP group had not been introduced into the
POM skeleton. In order to probe the effect of the pH, according
to the protocol of step ③, only the pH was changed from 3–4 to
2–3 (step ⑤), no crystal was obtained. Keeping the molar ratio
of the starting materials as step ②, and pH adjusted to 2–3, the
blue block crystals (compound 1) were successfully harvested
(step ④). The above results indicate that the {(C6H5P)2W5} com-
polyanions and [(Cu(H2O))2(μ-bipy)2]4+ cations, and the asym-
metrical unit of 1 is shown in Fig. S1a.† The {(C6H5P)2W5} unit
acts as a tetradentate ligand and coordinates with four Cu2+
ions via four terminal oxygen atoms (O2/O2′, O7/O7′) (Fig. 1a),
and the symmetrical Cu1 is five-coordinate with two N atoms
(N1, N2) from two bipy ligands, two terminal oxygen atoms
(O2, O7), and one terminal water ligand (O1W), to give a
square-pyramid configuration. The bond lengths of Cu1–O2,
Cu1–O7, Cu1–N1, Cu1–N2 and Cu1–O1W are 1.961(9), 2.363
(9), 2.008(10), 2.035(10) and 1.963(10) Å, respectively. The W–O
distances are in the normal range (1.705(9)–2.395(8) Å)
(Table S1†). As shown in Fig. 1b, the adjacent two
{(C6H5P)2W5} units are linked by two symmetrical Cu2+ (Cu1/
Cu1′) ions, forming an infinite 1-D chain stretching along the
c axis. Obviously, the Cu2+ ions not only link {(C6H5P)2W5}
units, but also connect bipy ligands, to generate polymeric 1-D
{Cu-bipy}n chains parallel to the b axis (Fig. 1c), with adjacent
chains interconnected by polyoxoanions into a 3-D framework
through coordination bonding and hydrogen bonding inter-
actions (Fig. 1d). The hydrogen bonds of O1W–H1WA⋯O5 and
O1W–H1WB⋯O8 are 2.707(13) and 2.671(12) Å (Table S2†),
respectively. The 3-D network topology of compound 1 can be
further described as in Fig. 2. In this simplification, the Cu
atoms act as one kind of four-node (4·64·8), and the
{(C6H5P)2W5} unit acts as the other (42·62·82) (Fig. 2a). From
the topological point of view, the 3-D structure of compound 1
can be simplified to a unique bimodal four-connected 4,4-c
net with stoichiometry (4-c)(4-c)2 and the point symbol for the
net is (4·64·8)2(42·62·82) (Fig. 2b).
For the structural unit of 2, as shown in Fig. 3 and S1b,†
each molecular unit is composed of one distorted metatung-
state [α-H2W12O40]6− polyanion, two Cu-bipy complex frag-
ments and twelve crystal water molecules. The four central μ-O
atoms of {W12} are disordered over eight positions with each
oxygen site half-occupied. Each {W12} coordinates to six Cu2+
ions via six terminal oxygen atoms (O1/O1′, O3/O3′, O4/O4′)
acting as a hexadentate ligand (Fig. 3a). The two crystallo-
graphically independent Cu(II) ions have similar coordination
environments. The Cu1/Cu2 centers coordinate with two WO6
octahedral oxygen atoms from two polyanion units, and four
nitrogen atoms from two bipy ligands. The Cu–O bond lengths
are 2.308(17), 2.308(17) for Cu1 and 2.446(16), 2.478(17) Å for
Cu2, respectively, and the Cu–N distances are in the range of
2.02(2)–2.15(2) and 2.04(2)–2.09(2) Å for Cu1 and Cu2, respect-
ively (Table S3†). A 3-D architecture is also constructed
through coordination and hydrogen bonding interactions
(Fig. 3b). As shown in Fig. 3b, each {W12} cluster is surrounded
Scheme 1 Schematic diagram of the pH- and the molar ratio-
controlled assembly process of compounds 1 and 2.
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Fig. 1 (a) Structural representation of the coordination environment of four Cu(II) ions around the Strandberg-type {(C6H5P)2W5} cluster in 1; (b) the
1-D chain of {(C6H5P)2W5} units linked by Cu-bipy/H2O fragments along the c axis in 1; (c) wire representation of the polymeric 1-D chains of {Cu-
bipy}n along the b axis (H and polyanions have been omitted for clarity); (d) wire representation of a 3-D structure of 1.
point symbol is (4·4·4·4·4·4·5·5·5·5·52·64·64·72·72) for the Cu
node and (4·4·4·4·4·4·52·52·52·52·52·52·716·716·716) for the
bimodal four-connected 4,4-c net with stoichiometry (4-c)(4-c)2
and the point symbol for the net is (4·64·8)2(42·62·82) {W12}
polyanion node (Fig. 4a), thus giving a short point symbol for
net (46·55·62·72)3(46·56·73) topology (Fig. 4b).
Fig. 2 (a) Perspective views of the four-connected (Cu atom and
Characterization
{(C6H5P)2W5} polyanion) nodes in compound 1. Blue dashed lines illus-
trate the four-connected circumstances of the Cu atom; (b) schematic
representation of the bimodal four-connected 3D net of
(4·64·8)2(42·62·82) topology (H atoms are omitted for clarity).
IR spectroscopy. The IR spectra of 1 and 2 are shown in
Fig. S2.† For compound 1, the peaks at 3447 and 3115 cm−1
are attributed to the characteristic vibrations of O–H and N–H.
The bands in the 1160–1033 cm−1 region are due to the v(P–O)
vibration of the organophosphorus. And the bands at
by six {Cu-bipy} clusters, and each {Cu-bipy} cluster actually 1613–1223 cm−1 are associated with the pyridine ring of a bipy
consists of two intertwined chains (Fig. 3c and d). From the molecule.2c The characteristic bands at 964 and 939 cm−1
topological viewpoint, the Cu atom can be reduced to a six- belong to v(W–Oterminal), and those at 887–560 cm−1 are
connection node, and the {W12} polyanion acts as one kind of believed to be v(W–Obridging), respectively.8 As for compound 2,
a six-connected node. The final net is a two-nodal six-con- the characteristic vibrations of the Keggin unit appear at
nected 6,6-c net with stoichiometry (6-c)(6-c)3. The extended 919, 864 and 775 cm−1 v(W–Oterminal) and v(W–Obridging),
6−
Fig. 3 (a) Structural representation of the coordination environment of six Cu(II) ions around the [H2W12O40
]
polyanion in 2; (b) polyhedral and
stick representation of the 3-D structure of 2; (c) stick representation of {Cu-bipy}n; (d) ball and stick representation of two intertwined chains con-
structed by {Cu-bipy}n around the periphery of polyanion clusters (the H and polyanions have been omitted for clarity).
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5b†). The absorption bands in the UV range of compounds 1
and 2 were assigned to a charge-transfer transition from the
O → W and the characteristic absorption of bipy.1f While the
absorption in the region of 500–800 nm are due to the d–d
transition coming from the introduction of the transition
metal Cu2+.
Solid-state fluorescence properties
The solid-state luminescence properties of 1, 2, the parent
compound TBA-(C6H5P)2W5, bipy and C6H5PO3H2 have been
detected at room temperature (Fig. S6†). From the result
shown in Fig. S6b,† it is found that the TBA-(C6H5P)2W5 shows
luminescence with two main emission peaks at 380 nm and
470 nm with λex = 290 nm, attributed to LMCT (O → W).
C6H5PO3H2 and bipy display an emission peak at about 292
and 363 nm (λex = 290 nm), respectively, assigned to the intra-
ligand π* → π transition (Fig. S6c†). Compared with TBA-
(C6H5P)2W5, compound 1 shows different luminescence pro-
perties: two main broad emission bands at 400 and 470 nm
with λex = 290 nm (Fig. S6a†), and the emissions of 1 are
mainly caused by the coordination of {(C6H5P)2W5} to Cu2+
ions.9 Compound 2 shows emission peaks at 450 and 470 nm
with λex = 290 nm (Fig. S6d†), compared with 1, the peak shifts
about 50 nm toward the longer wavelength.
Fig. 4 (a) Perspective views of the six-connected (Cu atom and {W12
}
polyanion) nodes in compound 2. Blue dashed lines illustrate the six-
connected circumstances of the Cu atom. (b) Schematic representation
of the bimodal six-connected 3D net of (46·55·62·72)3(46·56·73) topology.
respectively. No v(P–O) vibration illustrates that the polyanion
is a hollow Keggin structure. The strong bands at 1614, 1399
and 1222 cm−1 are characteristic vibrations of bipy, and the
bands at 3456 and 3136 cm−1 are due to O–H and N–H
vibrations.2c
XRPD. The XRPD patterns of compounds 1 and 2 as well as
their simulated XRPD patterns are shown in Fig. S3.† The
diffraction peaks on the patterns correspond well in position,
confirming that the product is in a pure phase. The differences
in reflection intensity are probably due to the preferred orien-
tation in the powder samples.
Catalytic activity of two compounds
The carbonyl protection reaction is one of the very important
organic reactions, and the acid catalyst plays a significant role
in this reaction. The synthesis of cyclohexanone ethylene ketal
was selected as a model reaction to research the acid catalytic
activity of compounds 1 and 2 (Scheme S1†).10,11 In order to
obtain the optimal catalytic conditions of the above target
reaction, the effects of various factors including reaction time,
amount of catalyst and molar ratio of cyclohexanone to glycol
were systemically investigated and selected. Using compound 1
as a representative example, the catalytic performances are pre-
sented in Fig. S7–9.† As seen from Fig. S7–9,† the optimum
conditions for the synthesis of cyclohexanone ethylene ketal
were as follows: the reaction time is 3.5 h, the molar ratio of
the catalyst to cyclohexanone is 1 : 350 (that is, 0.06 mmol cata-
lyst); the cyclohexanone/glycol molar ratio is 1 : 1.4; reaction
temperature 95–100 °C, respectively. For comparing the cata-
lytic activities of the as-synthesized compounds with the refer-
ence catalysts, the target reaction catalyzed by CuCl2, TBA-
(C6H5P)2W5, bipy-Cu-(C6H5P)2Mo5 (entries 2–4) was also per-
formed under the optimum conditions, and the results are
listed in Table 2. In addition, the reaction without any catalyst
was also tested, and the yield of ketal was only 9% (entry 1). As
TGA–DTA. To study the thermal stability of compounds 1
and 2, their thermal behaviours were investigated using
thermal analysis methods. TG and DTA analyses of 1 and 2 are
given in Fig. S4.† From Fig. S4a,† it is found that the TG curve
of compound 1 displays a three-continuous weight loss. The
total weight loss of 26.11% (calc. 25.82%) in the temperature
range of 35–550 °C corresponds to the losses of two coordinate
water molecules, two bipy molecules and two phenyl groups.
Two exothermal peaks observed at 400 and 482 °C in the DTA
curve are assigned to the combustion of the phenyl groups
and bipy molecules, which means that the skeleton of the
Stranberg-type POM is retained below 550 °C. For compound
2, a two-continuous weight loss step is observed (Fig. S4b†).
The total weight loss of 28.25% (calc. 27.51%) at the tempera-
ture 37–530 °C is attributed to the losses of twelve water mole-
cules, two H+ ions and six bipy ligands. One exothermal peak
observed at 515 °C in the DTA curve is assigned to the combus-
tion of the bipy molecules. The above results show that com-
pounds 1 and 2 have good thermal stabilities.
Solid UV-vis absorption spectra
In order to further understand the optical properties of the exhibited in Table 2, it is found that compounds 1 and 2
two compounds, the solid UV-vis absorption spectra of 1, 2, showed high acid catalytic activity, and the yield of cyclohexa-
parent compound TBA-(C6H5P)2W5 and bipy, between 200 and none ethylene ketal is 95% and 91%, respectively (entries 5
800 nm, are presented in Fig. S5.† As seen from Fig. S5c and and 6). Obviously, compound 1 exhibits better catalytic activity
5d,† the parent compound TBA-(C6H5P)2W5 and bipy have than the parent TBA-(C6H5P)2W5, essentially, the Cu2+ and W6+
strong bands only in the UV region (227 nm and 260 nm), moieties, as the catalytically active center, play the most impor-
while the two inorganic–organic hybrid title compounds show tant roles in the catalytic reaction. Furthermore, the acid cata-
strong bands in the UV region and visible region (Fig. S5a and lytic activity of compound 1 is slightly higher than that of
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with the isostructural bipy-Cu(II)-{(C6H5P)2Mo5} coordination
polymer,2c compound 1 displays higher catalytic performance
toward the synthesis of cyclohexanone ethylene ketal, and can
be reused for three cycles without significant deactivation. The
solid-state luminescence study shows that compound 1 also
displays good photoluminescence properties.
Table 2 Catalytic performances of 1 and 2 compared with the refer-
ence catalysts for the synthesis of cyclohexanone ethylene ketal
Entry
Catalyst
Solubility
Time (h)
Yield (%)
a
1
2
3
4
5
6
—
—
3.5
3.5
3.5
2.5
3.5
3.5
9
∼100
73
94
95
a
CuCl2
TBA-(C6H5P)2W5
Soluble
Insoluble
Insoluble
Insoluble
Insoluble
a
b
bipy-Cu-(C6H5P)2Mo5
Compound 1a
Compound 2a
91
Acknowledgements
Reaction conditions: the molar ratio of cyclohexanone to glycol was
1 : 1.4 (0.1 mol of cyclohexanone); water-carrying agent: 10 mL of
cyclohexane; reaction temperature: 95–100 °C. a The molar ratio of the
catalyst to cyclohexanone is 1 : 350. b The molar ratio of the catalyst to
cyclohexanone is 1 : 200.
This work was financially supported by the Natural Science
Foundation of Liaoning Province (no. 2013020128), the Foun-
dation of Education Department of Liaoning Province (no.
L2013414), the National Natural Science Foundation of China
(no. 21503103), the National Undergraduate Innovation and
Entrepreneurship Training Program (no. 201510165039) and
the Youth scientific research project of Liaoning Normal Uni-
versity (no. LS2014L018).
Notes and references
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Fig. 5 The catalytic activity of 1 (a) and 2 (b) used after four runs.
compound 2. The difference in the catalytic activity of com-
pounds 1 and 2 may be derived from the differences in their
structures. The former has an organophosphorus center in the
structure, which may have a positive effect on the acid catalytic
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compound 1 and bipy-Cu-(C6H5P)2Mo5, the dosage of 1 is less
than that of bipy-Cu-(C6H5P)2Mo5 under the same reaction
conditions, in other words, the catalytic activity of the
{(C6H5P)2W5} series is better than that of the {(C6H5P)2Mo5}
series. As shown in Fig. 5, compounds 1 and 2 can be reused
for three cycles without distinct decrease of the ketal yield
under optimum reaction conditions. In particular, the catalysts
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catalysts for the synthesis of cyclohexanone ethylene ketal.
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In summary, a new Strandberg-type organophosphotungstate
has been successfully assembled. Compound 1 represents the
first example of transition metal introduced into the {(RP)2W5}
series, which enriches the structural diversity of the Strand-
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Dalton Trans.